The invention relates to a magnetic resonance (MR) apparatus that is provided with an open magnet system for generating a uniform, steady magnetic field in an examination zone, and with a quadrature coil system for generating or for detecting RF magnetic fields in the examination zone. An “open” magnet system is to be understood to mean herein a magnetic system that generates a (usually vertical) magnetic field between two pole plates of a permanent magnet or between two coil packages of an electromagnet. The examination zone is then open to a high degree, as opposed to the situation in the case of a “closed” magnet system that encloses the examination zone as a cylinder. The invention also relates to a quadrature coil system for such a magnet system.
DE 4 232 884 already discloses an MR apparatus provided with an open magnet system and a coil system for receiving or generating RF magnetic fields. The coil system consists of two similar resonators that are offset relative to one another in the direction of the main magnetic field and are situated to both sides of the examination zone, said resonators being tuned by way of tuning capacitances. Each resonator includes a conductor of large surface area that at the same time serves as an RF shield, and a conductor structure that includes a number of strip-like conductors that are situated in the same plane and are arranged at a distance from one another.
A conductor structure of this kind is only capable of generating or receiving a linearly polarized RF magnetic field in the examination zone. However, it is also possible to supplement each resonator with a second conductor structure that is rotated through 90° relative to the first conductor structure, thus forming a quadrature coil system that generates a circularly polarized field. The two conductor structures must then be situated very near one another, thus giving rise to electrical fields and hence undesirable capacitive coupling between the conductor structures, which phenomena must be compensated again by one or more inductances.